1. Field of the Invention
The present invention relates in general to differential drivers and in particular to a differential driver having multiple output voltage ranges.
2. Description of Related Art
FIG. 1 illustrates a well-known prior art differential driver 1 for responding to a differential input signal (INPUT) by producing a differential output signal (OUTPUT) having adjustable voltage levels. Driver 1 employs a pair of transistors QA and QB having emitters connected to a current source 2 and collectors separately coupled to the output of a digital-to-analog converter (DAC) 3 through a matching pair of resistors RA and RB. The output voltage of a DAC 4 controls the amount of current flowing through source 2. The INPUT signal is applied across the bases of transistors QA and QB. When INPUT+ swings high and INPUT- swings low, transistor QA turns on and transistor QB turns off. Transistor QA directs the current drawn by source 2 through resistor RA. The resulting voltage drop across resistor RA pulls OUTPUT+ to a low level (V.sub.LOW). Resistor RB pulls OUTPUT- up to the output voltage V.sub.HIGH- of DAC 3. Conversely, when INPUT- swings high and INPUT+ swings low, transistor QA turns off allowing OUTPUT+ to rise to V.sub.HIGH and transistor QB turns on pulling OUTPUT- down to V.sub.LOW. Input data D.sub.HIGH to DAC 3 sets the level of V.sub.HIGH. Input data D.sub.LOW to DAC 4 sets the amount of current through current source 2, thereby setting the level of V.sub.LOW.
Various factors limit the speed of operation of driver 1. Inherent capacitance at the collectors of transistors QA and QB, together with the load impedance of RA and RB cause voltage swings in the OUTPUT signal to exhibit RC exponential charging behavior rather than to change abruptly in response to a state change in the INPUT signal. The resulting lag between state changes in the INPUT and OUTPUT signals limits the speed at which driver 1 can operate. The OUTPUT signal slew rate can be increased by reducing the resistance of RA and RB and increasing the current drawn by source 2, but only at the cost of an undesirable increase in the driver's power consumption.
The well-known Miller effect can also limit the operating speed of driver 1. When driver 1 requires a substantial gain G in order to provide a desired output voltage range, the inherent collector-base capacitance C.sub.cb of transistors QA and QB appears to the INPUT signal to be a base-ground capacitance G+1 times as large as C.sub.cb. This apparent input capacitance, in connection with the drive impedance behind the INPUT signal reduces the INPUT signal's slew rate and therefore limits the driver's switching speed. The Miller effect can be reduced by reducing the drive impedance and increasing the base current, but this also increases power consumption. The Miller effect can also be reduced by limiting INPUT signal swing, but only at the cost of reducing OUTPUT signal range.
At the low end of the OUTPUT signal voltage range noise in the INPUT signal coupled through the QA and QB collector-base capacitances can become an excessively large component of the OUTPUT signal. The collector-base capacitance of transistors QA and QB also causes "preshoot" in the OUTPUT signal which can also limit driver operating speed. For example, when the INPUT+ logic signal goes low, the collector of transistor QA goes low briefly before transistor QA turns off. Since the collector voltage is initially driven below V.sub.LOW charging current passing through RA after QA turns off takes longer to supply enough charge to the collector capacitance to bring the collector voltage up to V.sub.HIGH. At the low end of the OUTPUT signal range, this preshoot effect becomes a significant factor in limiting slew rate.
The resolution with which driver circuit 1 can adjust the low logic level V.sub.LOW, of its OUTPUT signal is entirely a function of the resolution of DAC 4. To increase the driver's resolution we must increase the resolution of DAC 4, but high resolution DACs are expensive.
What is needed is a high speed wide range differential driver having output signal voltages that can be adjusted with high resolution without requiring the use of a high resolution DAC.